1. Field of the Invention
This invention relates to devices for controlling or limiting differences in angular velocity or angular acceleration between relatively rotatable members. Hereinafter, and in the appended claims, such a device is referred to as "a control coupling". Control couplings find use, inter alia, in controlling the relative rotation between parts of a differential gear, particularly an inter-axle differential gear in a four-wheel drive vehicle. The invention also relates to a four-wheel drive vehicle having an inter-axle differential gear associated with a control coupling.
2. Description of the Prior Art
Many proposals have been made for viscous shear couplings in which there are two relatively rotatable elements, one of which may be a housing in which the other element is rotatable, the housing containing a certain amount of viscous fluid. Normally the housing is less than half filled with viscous fluid but when the coupling rotates, the viscous fluid is thrown outwardly by centrifugal force so as to provide a film of viscous liquid between at least the outer circumferential parts of the two elements. Often these couplings are intended to disconnect when a certain speed difference is reached and it has been proposed that when this speed difference is reached the effect of the centrifugal force will be such as to compress elements around the outside of the housing thus to allow the viscous fluid to move from between the plates and to disconnect the coupling.
It has also been proposed to have couplings which are temperature sensitive and which are particularly designed for driving the cooling fans of motor vehicle engines. Such couplings are associated with a temperature sensitive element in such a manner that if the temperature drops below a predetermined value the coupling is disconnected.